1. Field of the Invention
The present invention relates to an ion source, and more particularly to a repeller structure disposed in opposed relation to an electron emitting cathode within a plasma generating chamber of an ion source to reflect the electrons toward the cathode.
2. Description of the Background Art
In recent years, it has been studied to, in a plasma generating chamber of an ion source, ionize a source gas by a cathode to generate a plasma, and sputter a target material by the generated plasma to allow desired ion species to be contained in an ion beam.
Specifically, the target material is provided at a distal end of a repeller and held in a replaceable manner so as to allow stable generation of ion species, as disclosed in the following Patent Document 1. A detailed structure therefor comprises a tubular repeller, and a target member (slug) which is housed in a distal end of the repeller. The repeller has a stepped portion provided on an inner peripheral surface of the distal end thereof to protrude inwardly, and the target member has a lockable portion provided on an outer peripheral surface thereof and configured to be locked to the stepped portion. The target member is fixed inside the repeller by locking the lockable portion of the target member to the stepped portion of the repeller, and then screwing an externally-threaded block into a threaded portion formed on an inner peripheral surface of the repeller, from the side of an upper end of the repeller.
The above structure is configured such that the outer peripheral surface of the target member is locked by the repeller. However, considering that the repeller is disposed within the plasma generating chamber, i.e., a limited space, there is a restriction on the size of the repeller. Thus, the size of the target member to be housed in the repeller will be restricted, which poses a problem of difficulty in increasing an area of a sputterable surface (a sputterable surface area) of the target member.
The above structure in which the repeller is disposed around the outer peripheral surface of the target member has another problem of an increase in size of the repeller. Further, the threaded portion is provided on the inner peripheral surface of the tubular repeller to allow the externally-threaded block to be screwed thereinto. This results in structural complexity of the repeller, and is likely to cause increases in material cost and machining cost when the repeller is fabricated from a single member by cutting.
Moreover, the above structure is configured such that, with respect to electrons emitted from a cathode, the repeller is disposed around an outer periphery of the target member, which means that a member opposed to an electron emitting portion of the cathode is the target member. This causes problems of deterioration in electron reflection efficiency and thus deterioration in plasma generation efficiency.
Meanwhile, in connection with miniaturization of large scale integration (LSI), there is a need for a technique of shallowly forming a high-concentration and activated impurity layer. The following Non-Patent Document 1 reports that, when pre-amorphization is performed by Ga ion implantation, a p+ diffusion layer having a resistance reduced by 20% or more as compared to pre-amorphization performed by Ge ion implantation is formed, irrespective of heat treatment conditions. Therefore, there arises a need for an apparatus for implanting Ga ions into a substrate.
In this connection, the Patent Document 2 discloses an ion source designed to mix an organic metal gas (e.g., gaseous triethylgallium [Ga (C2H5)3]) with an inert gas, and generate an ion beam containing gallium ions.
However, the organic metal gas itself has a strong reactivity in many cases, and active molecules and active atoms generated by ionizing the organic metal gas also have a strong reactivity in many cases. Thus, an ion source configured to introduce such an organic metal gas directly into a plasma generating chamber has the following problems:
(1) Components, such as a filament, a repeller and an insulator, in the plasma generating chamber, are subjected to alteration, which causes a change in amount of generation of plasma (plasma generation amount) and thus in amount of generation of ion beam (ion beam generation amount), and a shortening of the usable life of the components;
(2) Contamination is more likely to occur in the plasma generating chamber. This contamination causes an insulation failure, for example, between the filament and the plasma generating chamber, to hinder a stable operation of the ion source; and
(3) In order to remove the contamination, it is necessary to frequently perform maintenance for the ion source. Moreover, the ion source using an organic metal gas and an inert gas as a source gas becomes structurally complicated.